1. Field of the Invention
The present invention relates to an optical network analyzer for measuring transmission characteristic, delay characteristic, etc. of light in various optical elements.
2. Description of Related Art
FIG. 15 is a block diagram showing a configuration of a conventional optical network analyzer. The conventional optical network analyzer has been disclosed by following references as the apparatus for performing chromatic dispersion measurement of an object to be measured in near infrared region. (Cf. S. Ryu, Y. Horiuchi, K. Mochiuki, “Novel chromatic dispersion measurement method over continuous Gigahertz tuning range,” J. Lightwave Technol., vol. 7, no. 8, pp. 1177–1180, 1989; M. Fujise, M. Kuwazuru, M. Nunokawa, and Y. Iwamoto, “Chromatic dispersion measurement over a 100-km dispersion shifted single-mode fiber by a new phase-shift technique,” Electron. Lett., vol. 22, no. 11, pp. 570–572, 1986)
The conventional optical network analyzer shown in FIG. 15 includes an optical measurement section 700, an object to be measured 200, and a network analyzer 750. The optical measurement section 700 includes a wavelength variable light source 702, an optical intensity modulator 704, a referential high frequency signal source 706, a photoelectrical converter 708, and an amplifier 710. The optical signal generated by the wavelength variable light source 702 is incidence on the object to be measured 200 after the optical intensity modulator 704 sinusoidally modulates intensity of the optical signal by a referential high frequency signal supplied from the referential high frequency signal source 706. Then, the optical signal which has transmitted the object to be measured 200 is converted into an electric signal by the photoelectrical converter 708, the electric signal is amplified by the amplifier 710, and is input into the network analyzer 750 as a measured signal.
The network analyzer 750 calculates phase shift of the measured signal by comparing the phase of the reference signal supplied from the referential high frequency signal source 706 and the measured signal input from the amplifier 710. A propagation delay time τ(λi) of the measured signal is expressed by the following equation using phase φ(λi, fIF).τ(λi)=φ(λi,fIF)/2πfIF
Where the wavelength of an optical signal is λi, and the optical intensity modulation frequency is fIF.
Therefore, the propagation delay time τ(λi) for every wavelength is calculated by measuring the wavelength of the light generated by the wavelength variable light source 702 with the wavelength of the light being changed continuously. A chromatic dispersion D(λi) is given by differentiating the propagation delay time with respect to the wavelength, and is calculated by the following equations.D(λi)=Δτ(λi)/ΔλiWhere,Δτ(λi)=τ(λi+1)−τ(λi)Δλi=λi+1−λi
FIG. 16 is a block diagram showing a configuration of the conventional image detection apparatus. The conventional image detection apparatus shown in FIG. 16 is an apparatus for imaging a tomogram of an object to be measured, such as a biological sample, disclosed by Japanese Patent Laid-Open No. 2-150747 bulletin and Japanese Patent Laid-Open No. 2000-121550 bulletin.
The conventional image detection apparatus shown in FIG. 16 includes an optical measurement section 800 and an image processing apparatus 850. The optical measurement section 800 includes a laser light source 802, a lens 804, a lens 806, a beam splitter 808, a mirror 810, a mirror 812, an optical-frequency converter 814, a referential high frequency signal source 816, a beam splitter 818, and a photoelectrical converter 820. The laser beam generated by the laser light source 802 is irradiated to the object to be measured 200, only rectilinear component of the transmitting laser beam is detected by optical heterodyne detection with the photoelectrical converter 820 by utilizing the directivity of optical heterodyne detection. The output of the photoelectrical converter 820 is input into the image processing apparatus 850 including a demodulator, a computer, and an image displaying apparatus. The image processing apparatus 850 images an optical tomogram, using only the intensity of a transmitting laser beam as information, and does not measure phase information.
According to the conventional optical network analyzer shown in FIG. 15,
(1) since it employs direct detection system for converting the frequency of an optical signal to electric signal directly, signal-to-noise ratio is about 10–20 db lower than the signal-to-noise ratio when the heterodyne-detection system is employed. Therefore, when an object with heavy losses is to be measured, dynamic range becomes narrow and accuracy of the measurement is aggravated or even it is impossible to measure the object.
(2) Since not only the object to be measured 200 but also the optical intensity modulator 704 is provided between the wavelength variable light source 702 and the photoelectrical converter 708, the drift of the transmission characteristic of the optical intensity modulator 704 has direct influence on the accuracy of measurement of dispersion and the like.
(3) Since the optical intensity modulator 704 modulates intensity of the optical signal, bandwidth of spectrum of the optical signal being incidence on the device under test 200 is two times wider than the modulation frequency, and it is unable to obtain high wavelength resolution of the modulation frequency. That is, it is ideal that the spectrum of the optical signal to be incidence on the device under test is a coherent light having constant amplitude.
(4) In order to increase resolution of delay time, the wide-band optical intensity modulator 708 is required. There is a trade-off relation between the resolution described in (3) and the wide bandwidth.
According to the conventional image detection apparatus shown in FIG. 16,
(1) since the laser beam generated by laser light source 802 is input into the photoelectrical converter 820 after it has passed through the lens 804, the lens 806, the beam splitter 808, the mirror 810, the object to be measured 200, the optical-frequency converter 814, the mirror 812, and the beam splitter 818, a path becomes long and its loss becomes large.
(2) Since there are two paths of the laser beam, i.e., a path via the object to be measured 200 and a path via the optical-frequency converter 814, adjustment of optical axis is complicated.
(3) Since phase comparison is not done, the propagation delay time etc. cannot be measured.
(4) Since the wavelength of the laser beam generated by the laser light source 802 is not variable or the laser beam is not swept, neither information on the wavelength characteristics nor the wavelength distribution of the object to be measured 200 can be obtained.